Color management guide by Arnaud Frich

Generalites about colors Calibrate a screen Calibrate a printer Color management with Photoshop Books, links, DVD and VOD on color management

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Shim
Updated on November 24, 2016
 

What is calibration ?

Even if our eyes are wonderful partners to show us the world, they're unfortunately completely unable to help us calibrate our color workflow. However, achieving a good color management between the pictures displayed on our monitor and their prints or when sharing them on internet, cannot be done unless you go through the calibration process. At the very least, I "highly" recommend you to calibrate your monitor, because it is relatively cheap and easy to do and also to calibrate your printer or to have it calibrated by a professional if you find it too expensive to do yourself.

 

 


Key points if you are a beginner ...

Here are the key points to remember on calibration for your color workflow management. The rest of this page will be dedicated to those who want to go into more details.

In order to have the color management work you need to calibrate your different devices : calibrate monitors, calibrate printers ... Thereby you get all the ICC profiles of each device.
Calibrating a device consists of two different steps :

  • Calibrate devices : that means placing your device in a known and stable way of working,
  • Characterize devices : thanks to monitor hardware calibrators or spectrometers for printer, you will precisely know how they behave (or in that case misbehave) and how to correct defects.

monitor calibration consists of using a colorimeter, also called hardware calibrator, to determine the right luminosity, contrast (this process is impossible to achieve with your eyes) - also called gauging phase -, and to have those defects being written by a software in an ICC profile (kind of identity card) - also called grading phase or calibration -.
In order to achieve this process, the software will display colors on the monitor that the colorimeter will measure.
A monitor colorimeter or hardware calibrator is therefore essential and remain cheap nowadays.

Printer calibration consists of choosing a paper, inks - the equivalent of the gauging process - print a reference color target and measure thanks to a spectrometer the reality of the colors printed.
In order to create the ICC profile of your printer, you just need the software to compare the printed colors to the colors that should have been printed.
Those calibrating kits are expensive, but you can go for the alternative of having it done by a professional for a limited amount of money.

Next key point : Calibrate a monitor suivre
Next key point : Calibrate a printer suivre


The whole color management process is based on this essential step: calibration and characterization of each device of your color workflow.
Thanks to calibration of each device, hence to the consideration of their color characteristics, you will be able to keep the "right" color, the L*a*b* color from one device correctly interpreted by another one.
Thanks to calibration you will obtain an ICC profile for each one of them.

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Calibrate / characterize: what for?

Probe to calibrate screenThe calibration process consists of two different steps: first calibration, then characterization. During the first step, you make sure the device runs at its best, at least in a known and stable way (it is called fixing the device to given working conditions) and during the second step, you are going to measure its characteristics - its color ID hence its distortions, would I be tempted to say, even if it is not completely right.
To characterize a device, you display known colors on the monitor, print a known color chart and then run a program controlling the measurement tool that will read them to create an ICC profile specific to this device in these working conditions. If you change contrast values on your monitor for instance, you'll modify its calibration and will have to go through charaterization again to create a new ICC profile.

Calibrate

It means aligning on a control measurement (standard meter, second of an atomic clock). For a monitor, for instance, it is to set once and for all the conditions in which your ICC profile will be generated, meaning luminosity, contrast, gamma and color temperature. For a printer, it is choosing ink, paper and printing definition.

Characterize

It means taking down the color characteristics of a device - screen, printer, etc., hence which L*a*b* color is displayed or printed when a known RGB or CMYK signal is sent. For a printer, it means making sure it is printing the "right" color, as if no deformation or defect had occurred - but of course, it is an image! The control tool in color management, is called a color chart for a printer and a light wave length for a monitor.

 

 

The aim of calibration and characterisation is thus to know precisely:

  • What the color characteristics of a device are or how a device is "distorting" colors in a more schematic way, even if it is not exactly what really happens - what color it really "sees" for a given RGB signal - in order to be able to "correct" it afterwards using an ICC profile that will induce adjustments compared to the standard;
  • AND what colors a device is able to reproduce - called color space or gamut - depending on their technical characteristics, meaning will a color be printable or displayable on a monitor?

For that, the ICC profile creation program sold with sensors and colorimeters will compare a colored chart which characteristics are precisely known, meaning the one and only L*a*b* color of each colored patch, to the color actually printed by the printer or to the light wave length displayed in RGB on the monitor.
Unfortunately, the calibration/characterization process can only be achieved with sensors or colorimeters, hence with accurate measurement tools. It is unfortunate for the cost, even if the situation has changed a lot and keeps on evolving.
In the end and after correction, in order to take into account the device's defects, you will have to, for instance, project a different quantity of CMYK inks for a standard eye to see a neutral grey on a print !

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Concrete example !

Example of a scanner calibration / characterization process :

 

Profil ICC

 

What we do is we make the scanner "see" colored patches - some of them neutral grey, while knowing the color characteristics of each one of them (CIE XYZ or L*a*b* values). However, when you visualize them in Photoshop on a calibrated monitor, they appear to have, in that example, a green color predominance, far from the original color!!
If you check the RGB values of the greys with the eyedropper tool, for instance, you will be able to check that they are not all the same - 128, 128, 128 should be displayed but 120, 138, 128 for a neutral grey is actually displayed. So this scanner sees green where it should see neutral grey !
That is why an ICC profile is dependent on a device and should not be chosen as workspace in Photoshop. An RGB value of 128, 128, 128 in the color space of this scanner appears to be slightly greenish. We then assign the right ICC profile of this scanner to this picture and everything goes back to normal regarding the way this one displays colors on the monitor. The ICC profile corrects the defects of the scanner.



 

So what is calibration for?

Calibration enables to know exactly how a device sees colors in order to be able to fix the differences with the standard thanks to color management tools. The ICC profile informs the rest of the color workflow of its own characteristics. By the way, we should be using the word characterization here as it is about marking down the characteristics of a device. So the necessary conclusion is:

CHARACTERIZE!

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How to?

Colorimeter and spectrophotometerIt is really important to stress here that the quality of human vision is such that it is possible to distinguish subtle variations when it comes to comparing two colors. A "standard" human eye is able to see a difference of 1/100 to 1/200 between two color nuances, as explained in the color page. But unfortunately, as for audition, really few people have an absolute vision. Almost no one is able, without a reference, to know which L*a*b* color he is looking at with only one color before his eyes.
It is impossible for a "standard" human eye to know, without any comparison support, if this red is a red of 700 nm of wave length, hence a precise kind of red in the L*a*b* color space. You will thus need a tool. A tool to measure or characterize.
As I said in the introduction, it would never come to your mind to put your finger in a test tube to evaluate the temperature of a liquid!

This tool is called a colorimeter or spectrophotometer and measures the wave length of the light. It is used to measure the colors a scanner x or y is able to digitize, a printer is able to print or a monitor is able to display. Charts and other references have been created for this purpose, accompanied by the program controlling the colorimeter and creating ICC profiles, that contains all the characteristics of the device.

Mire d'imprimanteWhat happens concretely during characterization? As an example, I am going to perform a printer calibration. First step will be to print with all the settings by default the color target - picture on the right - which is made of a multitude of known colored patches, that's to say, for each of them, the calibration software, which is also an ICC profile creator, knows the CIE XYZ coordinates.
The printer prints the chart as it is and delivers a print of this file. My ICC profile creation tool is now going to read, thanks to the colorimeter, the RGB values of each patch really printed and compare them to the expected L*a*b* colors. The difference between the L*a*b* color and the printed color is saved into the ICC profile matrix. From now on, for any picture printed on this device, with this paper and these inks, I will have Photoshop convert the RGB values of my image into RGB values compatible with my printer and its ICC profile. The image file to be printed will be transformed in order to take into account my printer characteristics, which I call defects!

Hopefully, once characterized, all devices are ready to be used as if they were natively able to replicate colors accurately. That being said, since it is a measurement, the quality of the ICC profile generated is directly related to the quality of the characterization device and to the software controlling it. It is only using his experience that a color management specialist will be able to say which system is efficient or not, since this device will deliver consistent results on different color workflows. In any case, even if those measuring devices are not perfect, they will always be better than a human eye, even an expert one!

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Temporary conclusion!

Nowadays, thanks to color management tools in Photoshop and calibration kits on the market, it is possible to get surprising results to achieve a work of quality, knowing what you're doing and possibly what you're missing without leaving it to chance. For instance, when the monitor is calibrated, you know where to look if you're experiencing a printing issue at home or with a lab printer, even if you do not have the ICC profile for this printer/paper/inks. Considering how much can be saved on paper, inks and above all time, I still believe those calibration kits are not so expensive, considering how they help. The great thing is the ability to share identical color files with others and, in any case, respectful of the quality of the pictures delivered by your digital camera !


To be remembered!

Calibration is a must! And nowadays, affordable and easy to perform.

At the very least, calibrate your monitor with a Spyder5Pro or a Colormunki Display sensor.

If it's not done already, avoid first price sensors which will deliver not great a result in the best case and a bad result most often.

Printer calibration can be done remotely for a small sum of money in comparison to the price of a calibration kit.


On the following pages, I will go through the calibration of each device, starting with the most important one, the monitor : monitor calibration Suivre


Next chapter : calibrate your monitor

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